Electric power facilities are required to have high reliability, and a technique for detecting an electrical anomaly in an early stage and evaluating its state is demanded. At the present, when an electrical insulation anomaly of a device occurs, an attempt is made to detect an electromagnetic wave radiated from discharge caused by the insulation anomaly, and to make a determination as to whether or not the device is anomalous and/or evaluate its state on the basis of information of the detected waveform. In particular, an UHF method which detects electromagnetic waves in the UHF band has drawn people's attention.
In a gas insulated device such as a gas insulated switchgear (GIS) or a gas insulated transmission line (GIL), a space between the high-voltage center conductor and the metal ground tank is usually exposed to an electric field of high intensity. Therefore, when a partial discharge occurs in such a device, the insulation between the center conductor and the metal tank may be broken; i.e., dielectric breakdown may occur, eventually. In view of this, there has been employed a diagnosis method of detecting an electromagnetic wave signal propagating within a gas insulated device in a stage of partial discharge, which is a harbinger of dielectric breakdown, and determining whether or not the electromagnetic wave signal is a signal attributable to the partial discharge (hereinafter referred to as a “partial discharge signal”), to thereby detect an insulation anomaly in advance. One of various proposed methods for detecting such a partial discharge signal is a UHF (Ultra High Frequency) method, which is considered to be the most useful for enhancing the reliability of such insulation diagnosis. In the UHF method, a high frequency electromagnetic wave of the UHF band (300 MHz to 3 GHz) is detected by an antenna which has sensitivity in this band.
Localization of a source which generates an electromagnetic wave originating from a discharge is performed by obtaining the position coordinates of a discharge source and the distance from an antenna (see Patent Documents 1 and 2). Patent Document 1 discloses a radio interferometer system which can obtain the arrival angle of an electromagnetic wave from an electromagnetic wave generation source. In the case where the electromagnetic wave source is located in a horizontal plane, the radio interferometer system obtains the arrival angle through use of two antennas. In the case where the azimuth and elevation angles are also needed or in the case where the position coordinates of the electromagnetic wave source are also needed, a plurality of antenna pairs are used. Namely, three or more antennas are disposed in order to obtain the three dimensional coordinates of the generation source and the distances from the antennas. The three dimensional coordinates and the distances are obtained on the basis of differences in arrival time among the antennas (point localization).
In the case where a building is present between a generation source and a detection sensor, the following problem occurs. Even when an electromagnetic wave in the UHF band is generated behind the building, diffraction allows detection of the radiated electromagnetic wave in the UHF band. However, since the diffracted wave is influenced by diffraction and scattering caused by the building, its waveform differs from the original waveform. Therefore, in the case where the generation position or state of a discharge is evaluated on the basis of a diffracted wave, there arises a problem in that the generation position or state cannot be evaluated correctively. Therefore, discrimination between a diffracted wave and a direct wave is important, and a desire has arisen to know or determine whether a detected waveform is a diffracted wave or a direct wave. The accuracy of diagnosis and the reliability of the results of diagnosis can be enhanced by the knowledge of whether a diagnosis technique is applied to the waveform of a diffracted wave or the waveform of a direct wave.